How does the dyno calculate cars power and torque?
The dyno, if it is set up correctly, knows the reduction ratio of the wheels. There should be a device attached to the engine that measures engine revs. (as well as a/f ratio) so if the dyno run is done in 1st gear, the back wheels are spinning at 540rpm, and the motor is spinning at 6000 rpm, the dyno knows that the reduction ratio through the drive train including the wheels and tyres is 9:1 (in this example, a 3:1 1st gear and a 3:1 rear, with a wheel radius of exactly 1 ft). It then measures the torque of the motor by determining the rate of change of the speed of the roller, and compares it to the weight, which is a fixed constant, and determines torque output. This torque output will be massive in 1st gear, but the true reflection of how much torque is being made is done when the reduction is factored in to it. So, if it puts out 2700ft-lbs at the wheels at 6000rpm, it is in actual fact making 300ft/lbs, as whatever figure it measures is divided by the reduction ratio, in this case 9:1. From this torque output, horsepower is calculated using the old chestnut of hp=tq*rpm/5252.
So the figure that you get given on the printout is not what it actually measured, but what it calculated.
Yes your car is putting out 1000's of ft-lbs in first gear, which is what enables it to move. if your car weighs 3000lbs, and puts out 3000ft-lbs in 1st, it can only accelerate at 1ft/sec^2.
It makes no difference what gear you do the run in, as the dyno *SHOULD* calculate the reduction ratio automatically, and so the result will be the same in every gear, although the actual amount of torque that is created at the wheels changes with every run, the calculated torque is divided by the reduction ratio and gets the same result.
The reason most runs are done in 4th gear is because in lower gears, the acceleration is much greater, which means there is a higher chance of wheel spin, which will create a false result, as well as being dangerous. Also with most cars having close to a 1:1 ratio in 4th, it means there is less percentage of error in the readings, as the reduction ratio is not so great.
1) Why does the dyno printout say I made my run from 20 to 120mph and not display the revs of my engine, and how can I work out the revs that my engine made power at?
2) Why does My Honda Make 900 ft-lbs at the Front wheels? Should I be entering it against Stephen Papadakis?
1) The dyno only told you the speed that the wheels were spinning at because the dyno operator did not set the dyno up correctly before doing your runs. This is most often done on “Dyno Days” where many cars are doing runs on the same day, and they really cannot be bothered with attaching some sort of RPM monitoring device to your engine. The way to calculate it is to look at the point at which your power and torque curves intersect. This point is 5252 Rpm. Then look down at the speed corresponding to those revs. So if at 5252 RPM, you are doing 105 mph just divide 5252/105=50, so 50 is your multiplication factor. Just multiply your speed by 50 to get your revs. (Okay so its not the most scientific method, but it works reasonably well, and is fairly accurate)
2) Most often when you hear about cars like Honda Accords getting results of 800 or 900 ft-lbs at the wheels, it is because the dyno operator is too lazy/inept to attach a rpm monitor to the engine, which then means that the dyno thinks the entire drive train is at a 1:1 ratio, and so it thinks that the motor is spinning at the same rate as the wheels when the power out-put is recorded. The dyno operator just relies on the car having a 1:1 4th gear, and doesn’t care about the diff ratio. All results like this should be dived by the diff ratio and the wheel radius (in inches)/12.
For example. My Accord dynoe'd at 624 ft-lbs at the wheels. A bit sceptical about this, I did some calculations.
4th gear is 0.98:1
my wheels have a radius of 11.5 inches
diff ratio is 4.261
624/(0.98*4.261*(11.5/12))
which gives me 155.9 ft/lbs, which sounds correct, bearing in mind the car makes 164 ft-lbs in its stock form at the crank.
So if the dyno is so wonky, how come it can work out the correct power reading?
The reason why the power out put is still correct is because the dyno calculates power based upon what it thinks is the correct rpm. To explain this, take the example of a car with a 4:1 first gear, 2.5:1 rear gear, exactly 1ft wheel radius. If this car puts out 300ft-lbs @ 5250 rpm, the dyno will think that the engine is only spinning at 525rpm, and putting out 3000ft-lbs. Even though it has not been calibrated correctly, it will use hp=tq*rpm/5250, and get the correct result. It will simply show an incorrect reading of torque and revs, (which is usually hidden by only displaying speed in kph or mph along the bottom) and show the correct power:
3000ft-lbs@525rpm=300hp
300ft-lbs@5250rpm=300hp
And in 90% of cases, the shape of the torque curve (which will always be correct) and the power output figure are the most important!!
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